1. Field of the Invention
The present invention relates to a sensor which detects NOx concentration in an exhaust gas from a combustion furnace, automobile engine, etc.
Currently, sensors based on a spectroscopic absorbance method, emission spectrochemical method, etc. are commercialized for the detection of NOx. However, based on these methodologies is large sized and expensive and also it takes a long time for the measurement due to its complexity of operation. Therefore, the development of a small sized and simple sensor is needed and a solid element sensor using a solid electrolyte has been proposed. The representative sensor hereto proposed is a semiconductor sensor which utilizes the fact that the electrical resistance of an oxide semiconductor changes by the existence of NOx gas or a solid electrolyte sensor which measures the equilibrium electromotive force caused by the difference of the partial presence of the gases on both sides of a separation wall.
In a solid electrolyte sensor, the selectivity and sensitivity are improved by the preparation of various kinds of nitrate electrode as a detecting electrode. For example, Japanese Patent Laid-open Publication No. Sho 61-184450 discloses a solid element sensor, wherein AgI or RbAg.sub.4 I.sub.5 is used as a solid electrolyte and the other electrode is coated by silver nitrate. This sensor is a concentration cell type, wherein Ag ion in the nitrate migrates in the solid electrolyte by the difference of - NOx concentration between the electrodes and causes an electromotive force which complies with Nernst formula and detects NOx concentration by the measurement of said electromotive force. This type of sensor is sensitive to either NO.sub.2 and NO, However the operation condition is limited because AgNO.sub.3 is water soluble and its melting point is 212.degree. C.
A concentration cell type sensor with high sensitivity to NO using NASICON (Na.sub.3 Zr.sub.2 Si.sub.2 PO.sub.12) as a solid electrolyte and NaNO.sub.2 as an electrode is disclosed in Chemistry Letters, vol.1, p.587.about.590 (1992). In this sensor, the working condition is limited due to the low melting point of NaNO.sub.2, 217.degree. C. and its deliquescence property.
As a sensor workable at comparatively higher temperature, a concentration cell using Na ion conductive .beta./.beta." alumina or .beta./.beta." alumina in which Na ion is replaced by Ba ion and using Ba(NO.sub.3).sub.2 or a mixture of NaNO.sub.3 and Ba(NO.sub.3).sub.2 as an electrode is disclosed in Denki Kagaku, vol.59, (1991), p.465.about.472. In these sensors, wherein Na ion conductive .beta./.beta." alumina is used as a solid electrolyte and Ba(NO.sub.3).sub.2 is used as an electrode, an electromotive force complying with the Nernst formula be obtained. In concentration cell type sensors wherein a mixture of NaNO.sub.3 and Ba(NO.sub.3).sub.2 is used and sensors, wherein .beta./.beta." alumina, in which Na ion is replaced by Ba ion and Ba(NO.sub.3 ).sub.2, is used it is reported that electromotive force complying with Nernst formula is obtained. Since Ba(NO.sub.3).sub.2 is a nitrate having higher melting point among nitrates, it is workable around 450.degree. C., However, there are issues for the measurement in an atmosphere containing water vapor and in long term stability.
Zirconia is used in a commercialized oxygen sensor as a thermal stable electrolyte at a high temperature. In an NOx sensor using zirconia, it is reported that a good performance is obtained by using mixed salts of Ba(NO.sub.3).sub.2 and or Ba(NO.sub.3).sub.2 with other salts as an electrode. (The 18th Chemical Sensor Meeting, vol.10, p.73.about.76, 1993). However, long range stability is an issue due to the deliquescence property of Ba(NO.sub.3).sub.2. A sensor using SnO.sub.2 which is stable at a relatively high temperature as an electrode and zirconia as an electrolyte is reported, However, the detectable gas is methane, but not NOx.
Thus, in a solid element type sensor using a solid electrolyte having a sensitivity to NOx, its working temperature is limited due to the melting point of nitrate used as an electrode. The working condition has been limited due to the destruction of the function of the sensor caused by melting and decomposition of nitrates when exposed to a temperature above the melting point. Furthermore, it was not possible to detect NO because it has an enough sensitivity to NO.sub.2, but almost no sensitivity to NO.